As known, keyboards are indispensable input devices of various electronic devices such as computers, mobile phones, personal digital assistants (PDAs), or the like. FIG. 1 is a schematic view illustrating the outward appearance of a computer keyboard according to the prior art. The surface of the computer keyboard 1 includes plural keys. These keys include ordinary keys 10, numeric keys 11 and function keys 12. When one or more keys are depressed by the user, a corresponding signal is issued to the computer, and thus the computer executes a function corresponding to the depressed key or keys. For example, when the ordinary keys 10 are depressed, corresponding English letters or symbols are inputted into the computer system. In addition, the function keys 12 (F1˜F12) can be programmed to cause corresponding application programs to provide certain functions.
With the maturity of computing technologies, the conventional keyboard 1 that has basic functions fails to meet the users' requirements. For this reason, the keyboard manufacturers make efforts in designing novel keyboards with diversified functions. Recently, a pressure sensitive keyboard having a pressure sensing function has been disclosed. In the pressure sensitive keyboard, a pressure sensing module is disposed under the plural keys for sensing the pressure exerted on the key that is depressed by the user. Generally, the pressure sensitive keyboard has a preset pressure value. In a case that the pressure exerted on the depressed key is lower than the preset pressure value, the pressure sensitive keyboard issues a lightly-depressed sensing signal. In response to the lightly-depressed sensing signal, the computer executes a lightly-depressed instruction. Whereas, in a case that the pressure exerted on the depressed key is higher than the preset pressure value, the pressure sensitive keyboard issues a heavily-depressed sensing signal. In response to the heavily-depressed sensing signal, the computer executes a heavily-depressed instruction. For example, in a case that a first pressure lower than the preset pressure value is exerted on the key A of the pressure sensitive keyboard, the computer executes a function of showing a small letter “a”. Whereas, in a case that a second pressure higher than the preset pressure value is exerted on the key A of the pressure sensitive keyboard, the computer executes a function of showing a capital letter “A”.
Hereinafter, the configurations and the operations of the conventional pressure sensitive keyboard will be illustrated with reference to FIG. 2. FIG. 2 is a schematic cross-sectional view illustrating a portion of a conventional pressure sensitive keyboard. As shown in FIG. 2, the conventional pressure sensitive keyboard 2 comprises a base 20, at least one key 21, a membrane switch circuit module 22 and a controlling unit (not shown). The key 21 is disposed over the base 20. The membrane switch circuit module 22 is arranged between the key 21 and the base 20. The key 21 comprises a keycap 211 and an elastic element 212. The keycap 211 is exposed outside the base 20 to be depressed by a user. As such, the keycap 211 is movable upwardly or downwardly with respect to the base 20. The elastic element 212 is disposed under the keycap 211. When the keycap 211 is depressed, an elastic force is accumulated in the elastic element 212, and the membrane switch circuit module 22 is depressed by the elastic element 212.
Please refer to FIG. 2 again. The membrane switch circuit module 22 is disposed under the key 21. The membrane switch circuit module 22 comprises a first-layer membrane circuit board 221, a second-layer membrane circuit board 222 and a spacing layer 223. The first-layer membrane circuit board 221 has a first contact 2211. The second-layer membrane circuit board 222 is disposed under the first-layer membrane circuit board 221. In addition, the second-layer membrane circuit board 222 has a second contact member 2221. The second contact member 2221 comprises a second left-side metallic contact 2221A, a second right-side metallic contact 2221B and a second graphite contact 2221C. The second graphite contact 2221C is arranged between the second left-side metallic contact 2221A and the second right-side metallic contact 2221B. The spacing layer 223 is arranged between the first-layer membrane circuit board 221 and the second-layer membrane circuit board 222. In addition, the spacing layer 223 has a perforation 2231. In the membrane switch circuit module 22, the first-layer membrane circuit board 221 and the second-layer membrane circuit board 222 are insulated from each other by the spacing layer 223. Due to the spacing layer 223, the first contact 2211 and the second contact member 2221 are separated and disconnected from each other if the keycap 211 is not depressed. The controlling unit may judge the strength of the pressure exerted on the depressed key 21. In addition, the controlling unit has a preset threshold voltage value corresponding to the preset pressure value.
In a case that the key 21 is depressed by the user with a first pressure lower than the preset pressure value, the keycap 211 is moved downwardly with respect to the base 20 to depress the elastic element 212, and thus an elastic force is accumulated in the elastic element 213. At the same time, the membrane switch circuit module 22 is depressed by the elastic element 212 and bent downwardly. In the membrane switch circuit module 22, the first-layer membrane circuit board 221 and the spacing layer 223 are moved downwardly and toward the second-layer membrane circuit board 222. In addition, the first contact 2211 of the first-layer membrane circuit board 221 is inserted into the perforation 2231, and then contacted with the second contact member 2221 of the second-layer membrane circuit board 222 to result in electrical connection. Since the strength of the first pressure is relatively smaller, the contact area between the first contact 2211 and the second graphite contact 2221C of the second contact member 2221 is relatively smaller. Due to the electrical connection between the first contact 2211 and the second contact member 2221, a first pressure-strength sensing voltage that is relatively lower is generated. By comparing the first pressure-strength sensing voltage with the preset threshold voltage value, the controlling unit may realize that the first pressure-strength sensing voltage is lower than the preset threshold voltage value. As a consequence, the controlling unit generates a lightly-depressed sensing signal indicative of a low magnitude of the first pressure. Whereas, when the keycap 211 is released and no longer depressed by the user, the compressed elastic element 212 is restored to the original shape and the elastic force is relieved. In this situation, the elastic element 212 is sustained against the keycap 211, and thus the keycap 211 is moved upwardly with respect to the base 20 and returned to the original position where no pressure is applied thereon.
Whereas, in a case that the key 21 is depressed by the user with a second pressure higher than the preset pressure value, the keycap 211 is moved downwardly with respect to the base 20 to depress the membrane switch circuit module 22, and thus the membrane switch circuit module 22 is bent and moved downwardly. In the membrane switch circuit module 22, the first-layer membrane circuit board 221 and the spacing layer 223 are moved downwardly and toward the second-layer membrane circuit board 222. In addition, the first contact 2211 of the first-layer membrane circuit board 221 is inserted into the perforation 2231, and then contacted with the second contact member 2221 of the second-layer membrane circuit board 222 to result in electrical connection. Since the strength of the first pressure is relatively larger, the contact area between the first contact 2211 and the second graphite contact 2221C of the second contact member 2221 is relatively larger. Due to the electrical connection between the first contact 2211 and the second contact member 2221, a second pressure-strength sensing voltage that is relatively larger is generated. By comparing the second pressure-strength sensing voltage with the preset threshold voltage value, the controlling unit may realize that the second pressure-strength sensing voltage is higher than the preset threshold voltage value. As a consequence, the controlling unit generates a heavily-depressed sensing signal indicative of a high magnitude of the second pressure. When the keycap 211 is released and no longer depressed by the user, the key 21 is returned to the original position. The operations of the conventional pressure sensitive keyboard have been described above.
During the operations of the conventional pressure sensitive keyboard, the pressure-strength sensing voltage is generated according to the contact area between the first contact 2211 and the second graphite contact 2221C of the second contact member 2221. After the pressure-strength sensing voltage is compared with the preset threshold voltage value by the controlling unit, the magnitude of the pressure exerted on the depressed key is realized. Generally, it takes a computing time for the controlling unit to judge the magnitude of the pressure exerted on the depressed key. In other words, the process of judging the magnitude of the pressure exerted on the depressed key wastes the computer's resources. Therefore, there is a need of providing a pressure sensitive keyboard without performing complex computation.